Optical isolators are required in particular in fiber-optic data transmission systems. The optical isolators are intended to prevent light reflected or diffracted at interfaces from impinging on optically active components, for example laser diodes or optical fiber amplifiers, and adversely affecting their operation.
An optical isolator is known from EP-B1-0 054 411. The optical isolator described therein essentially contains two wedge-shaped plates of an optically birefringent material, with a Faraday rotator disposed therebetween. The operation of the optical isolator is based on the observation that a parallel light beam impinging on the isolator from one side leaves the isolator as a parallel light beam on the other side, at which point the light can then be coupled into an optical waveguide by a lens. On the other hand, parallel light which impinges on the isolator from the other side, leaves the isolator as a divergent beam which cannot be coupled back in again if the optical waveguide and the imaging optics are arranged accordingly.
The optical isolator described in the above referenced patent document is independent of the polarization of the impinging light and is therefore particularly suited for applications in data transmission system with conventional non-polarization maintaining fibers. However, since light can be coupled into optical fibers only if the position of the respective imaging elements is precisely adjusted, the individual components of conventional optical isolators require complex active adjustments. The term "active adjustment" describes a process whereby the components have to be arranged and positioned in the optical light path until the light beam has the desired properties. This procedure is usually very time-consuming and labor-intensive; a measuring device has to be continuously monitored while the components are adjusted with the help of extremely accurate positioning devices. Because the high costs associated with this procedure, optical isolators of this type continue to be quite expensive and hinder the migration of fiber-optic data transmission systems to the end user.